1. Field of the Invention
This invention relates generally to thermal print heads which are widely used in facsimile machines, word processors and so forth. More specifically, the invention relates to a thermal print head of the type which comprises a head circuit board carrying a resistor line divisionally actuated by an array of drive IC's.
2. Description of the Prior Art
In general, a line-type thermal print head comprises an elongate head circuit board which carries a heating resistor line extending along one longitudinal edge of the circuit board. The circuit board also carries an array of drive IC's for divisionally actuating the resistor line to provide a line of multiple heating dots, and a printed conductor pattern including connection terminals disposed in a comb-like arrangement adjacent to the other longitudinal edge of the circuit board. The connection terminals are used to establish electrical connection to external circuitry.
In such a thermal print head, it is known advantageous to reduce the total number of connection terminals and to arrange them locally within a limited region which is substantially shorter than the head circuit board. The advantages of such an arrangement are fully described in U.S. Pat. No. 4,963,886 (Patented: Oct. 16, 1990).
Specifically, the above U.S. patent teaches that at least selected connection terminals are used commonly for at least selected drive IC's by utilizing printed wiring conductors, thereby reducing the required number of connection terminals. While this U.S. patent does not disclose a specific arrangement for the wiring conductors, it is conceivable to arrange the wiring conductors in such a manner as shown in FIG. 5 or 6 of the accompanying drawings.
In FIG. 5, a head circuit board 1' is shown to carry a heating resistor line 2' extending along one longitudinal edge of the circuit board, and an array of drive IC's 3a'-3c' for divisionally actuating the resistor line 2'. The circuit board is further formed with a printed conductor pattern which includes a wiring conductor pattern 4'-8b', a common electrode 9', and connection terminals 11'-15b'.
The wiring conductor pattern includes eight separate printed wirings 4'-8b' for the respective drive IC's 3a'-3c'. Specifically, the printed wirings consists of a power supply wiring 4' common for the respective drive IC's 3a'-3c', a clock signal wiring 5' also common for the respective drive IC's 3a'-3c', a latch signal wiring 6a' only for the central drive IC 3b', an additional latch signal wiring 6b' common only for the two drive IC's 3a', 3c', two kinds of enabling signal wirings 7a', 8a' only for the central drive IC 3b', and two additional kinds of inabling signal wirings 7b', 8b', common only for the two drive IC's 3a', 3c'. The respective wirings 4'-8b' are made to extend partially under the drive IC's 3b' , 3c'.
The connection terminals 11', 12', 13a', 13b', 14a', 14b', 15a', 15b' are connected to the respective wirings 4'-8b' and arranged centrally at suitable spacing adjacent to another longitudinal edge of the head circuit board 1' which is located opposite to the resistor line 2'. Thus, the number of these connection terminals 11'-15b' is the same as that of the respective wirings 4'-8b'.
The common electrode 9' is made to extend along the four edges of the head circuit board 1'. The common electrode itself has a pair of connection terminals 9a' which are arranged in side-by-side relation to the other connection terminals 11'-15b' to provide a total of ten connection terminals.
The connection terminals 9a' and 11'-15b' are used for connection to external circuitry through a flexible cable 16' which has a wiring conductor pattern 16a' in corresponding relation to the connection terminals.
According to the arrangement shown in FIG. 5, only the two wirings 4', 5' and their counterpart connection terminals 11', 12' are used commonly for all the drive IC's 3a'-3c'. This limitation comes mainly from the fact that the respective wirings 4'-15b' cannot cross each other in a same plane without electrically connecting to each other.
Thus, the arrangement of FIG. 5 cannot realize a drastic reduction in the total number of connection terminals, consequently failing to fully provide the intended advantages described in U.S. Pat. No. 4,963,886. Particularly, when the drive IC's 3a'-3c' are provided in a larger number than illustrated in FIG. 5, the total number of connection terminals will also increase correspondingly, which necessitates enlargement in the width W' of the flexible cable 16. As a result, the overall size, weight and cost of the thermal head will inevitably increase.
FIG. 6 shows another possible arrangement for a head circuit board 1". In FIG. 6, all elements are represented by reference numerals similar to those used in FIG. 5 but each accompanied by a double prime ("). Thus, the elements which are identical to those already described in connection with FIG. 5 are not described here to avoid substantial duplication.
According to the arrangement of FIG. 6, a total of eight connection terminals 9a", 11", 12", 13", 14", 15a", 15b" are provided. Of these connection terminals, the four terminals 11"-14" are connected commonly to all the drive IC's 3a"-3c" through four printed wirings 4"-7" which are made to extend along respective complicated paths without crossing each other. On the other hand, the terminal 15a" is connected only to the single drive IC 3a" through a printed wiring 8a", whereas the terminal 15b" is commonly connected only to the two drive IC's 3b", 3c" through a printed wiring 8b". Both of complicated paths.
Obviously, the arrangement of FIG. 6 is advantageous over that of FIG. 5 in that the total number of connection terminals 9a", 11"-15b" is additionally reduced together with the width of the flexible cable 16" which is connected to the connection terminals. However, realization of this arrangement requires that the respective wirings 4"-8b" are made to extend along the complicated and long paths in extremely crowded condition, so that the wiring patterning (etching) is impractically difficult and costly. Such a problem becomes more serious when the total number of drive IC's increases to necessitate further complication of the wiring pattern.
Though not illustrated, it is also conceivable to use a multi-layer head circuit board wherein various wirings in different layers are made to cross each other three-dimensionally without electrical connection therebetween. However, such a solution is even more costly than the arrangement of FIG. 6 due to the adoption of the multi-layer head circuit board which requires repetitive formation of different layers and repetitive etching on the different layers.